1. Field of the Invention
The present invention relates to semiconductor devices using semiconductor BCN compounds such as a light-emitting device and a solar cell.
2. Discussion of the Background
In recent years, various devices using semiconductor materials are used in the field of electronics.
For example, a semiconductor light-emitting diode is generally used as a light-emitting device for a display purpose. Likewise, a semiconductor laser is generally used for an optical communication and for information processing. The light-emitting diode and semiconductor laser, which are close to each other in the principle of light emission, are constructed to comprise a pn junction. To be more specific, electrons and holes are injected into the semiconductor layers forming the pn junction by applying a forward bias voltage across the element, with the result that these electrons and holes are recombined in the vicinity of the pn junction so as to emit light. When it comes to the light-emitting diode, spontaneous emission is radiated as a result of the recombination. On the other hand, when it comes to the semiconductor laser, the emission as a result of recombination is allowed to resonate between resonating planes so as to bring about an induced emission, thereby obtaining an oscillated light having uniform phase. The semiconductor laser is featured in that it can be miniaturized, can be operated with a high efficiency, and permits a rapid modulation.
However, the wavelength of the light emitted from the light-emitting diode or the semiconductor laser is limited by the band gap of the light-emitting layer, resulting in a failure to cover the entire wavelength region of the visible light. Semiconductor materials used in the semiconductor laser include, for example, InGaAsP, which is used in an infrared laser, and InGaAlP, which is used in a red laser. When it comes to a blue laser, GaN, ZnSe and ZnS are under study. However, these semiconductor materials leave room for further improvement in the crystal quality and lack in reliability. Further, the conventional semiconductor materials contain harmful elements such as As and Se and costly elements such as Ga and, thus, are not satisfactory in terms of environmental problems and manufacturing costs of the semiconductor laser. Such being the situation, vigorous studies are being made in an attempt to develop novel semiconductor materials which emit light of wavelengths covering a wide range.
What should also be noted is that a high importance is placed in recent years on the necessity of substitute energies for petroleum. Further, the environmental problems are turned serious. Under the circumstances, a solar cell utilizing sunlight, which is a safe and unlimited clean energy, has come to be used. Solar cells using semiconductor materials such as a single crystal, polycrystalline or amorphous Si or GaAs have come to be put into practical use to date. The solar cell includes as a basic construction a semiconductor pn junction or a semiconductor-metal Schottky junction. If the semiconductor layer acting as a light absorption layer is irradiated with sunlight, numerous electrons and holes are generated and accelerated in the vicinity of the junction so as to cause a flow of current. When it comes to an element of a pn junction type, the electrons and holes flow into the n-type and p-type semiconductor layers, respectively. Also, a voltage is generated through an external load resistor.
The solar cell is required to satisfy various characteristics. Particularly, it is important to improve conversion efficiency, i.e., a ratio of the output at an optimum operating point to the input sunlight. In many cases, an improvement of the conversion efficiency permits also improving other characteristics. In the case of a silicon solar cell, which is most widely used nowadays, a conversion efficiency of 24% has already been obtained in a laboratory level. However, the value in the practical level is not so high. Specifically, the conversion efficiency in the practical level is only about 20% even in a solar cell using a semiconductor material of GaAs/AlGaAs which is said to exhibit a high conversion efficiency.
Where a junction is formed between two layers, strain or stress is generated at the interface between the two layers because of a difference in lattice constant between the two layers. The particular strain or stress is one of the reasons for a failure to improve the conversion efficiency of the solar cell. Specifically, the strain or stress forms a deep energy level within the semiconductor layer so as to provide a recombination center between the electron and hole, leading to a low conversion efficiency. Naturally, it is very important to develop semiconductor materials and combinations of semiconductor materials and metal materials capable of forming a good junction interface.
Further, in many various semiconductor devices other than the light-emitting device and solar cell, a marked improvement in device characteristics can be expected by using semiconductor materials and combinations of semiconductor materials and metal materials having a suitable band gap and capable of forming a good junction interface.
Under the circumstances, the present inventors have paid attention to BCN compounds as novel semiconductor materials. It is possible for the BCN compound, which is represented by a general formula B.sub.x C.sub.y N.sub.z (x, y, z.gtoreq.0), to have two kinds of crystal structures, i.e., a cubic system and hexagonal system. In general, the crystal growth of the BCN compound is achieved by a CVD (chemical vapor deposition) method. Under the general conditions differing from the atmosphere of high temperatures and high pressures, the BCN compound grows into a crystal of hexagonal system having a layered structure. The properties of the BCN compounds having a layered structure depend on the combination of the component elements, ratio of the component elements, and arrangement of the atoms of the component elements. Depending on these factors, the BCN compounds exhibit various properties similar to those of metals, semiconductor materials having various band gaps, and insulators. For example, it is well known in the art that graphite consisting of carbon atoms alone exhibits properties of a semi-metal. Boron nitride (BN) is known well to be an insulator having a band gap of about 6 eV. Further, it has been confirmed by recent research that BC.sub.3 and C.sub.5 N are metals. Still further, it is reported by the present inventors that BC.sub.2 N is a semiconductor (J. Appl. Phys., Vol. 78, No. 4, pp. 2880-2882, Aug. 15, 1995). What should also be noted is that these BCN compounds have the same crystal structure and are close to each other in the lattice constant.
A semiconductor device having a BCN compound actually used therein is unknown to the art. However, it is considered possible in use effectively the BCN compounds in various semiconductor devices by utilizing the various properties of the BCN compounds pointed out above. In order to use the BCN compounds in the manufacture of a semiconductor device exhibiting desired characteristics, it is necessary to control as desired the compositions of the BCN semiconductor layers, to control the doping of p- and n-type impurities in the BCN semiconductor layers, and to suppress the defects in the BCN semiconductor crystals and junction interfaces.